Polycarbonate resins are generally suitable for molding medical equipment and devices. A variety of medical devices, including blood filters, blood oxygenators and the like, have been molded of polycarbonates. Prior to use, these medical devices must be sterilized, preferably by exposure to ionization radiation, e.g. gamma radiation or electron beam radiation, for a prescribed period of time. While the physical and mechanical properties of polycarbonates are substantially unaffected by this radiation, the exposure results in an often undesirable change in the color, a general yellowing, of the resin.
It has been the object of several researchers in the field to identify the means to reduce this objectionable formation of color. The relevant paent literature includes U.S. Pat. Nos. 4,804,692; 4,874,802 and 4,904,710 which disclosed gamma radiation resistant polycarbonate compositions containing polyether polyols.
Also relevant is 11.08.88-JP198940 which disclosed a polycarbonate composition containing an ester having mercapto group of thio glycolic acid or mercapto-propionic acid and alcohol capable of being sterilized by exposure to radiation. Additionally relevant is DE-OS 41 32 629 which disclosed a polycarbonate composition containing a stabilizing aromatic compound containing sulfonic acid ester substituents; the composition being stabilized against exposure to gamma radiation. A radiation sterilizable composition used for medical devices containing a semi-crystalline polymer, a hindered phenolic stabilizer, phosphite and a thioester (or thioether) has been disclosed in EP 303,894. Enhancing the color stability to sterilization radiation of polymer compositions has been disclosed in U.S. Pat. No. 4,939,185. Accordingly, compounds having defined thioether groups, including dithiane, are said to stabilize aromatic polycarbonate upon exposure to sterilizing radiation. Compounds having sulfone functional groups and thiazole functional groups have been disclosed to enhance the color stability of polycarbonate compositions in U.S. Pat. Nos. 4,880,855 and 4,880,856, respectively.
U.S. Pat. Nos. 4,067,846 and 3,909,490 disclosed flame retardant polycarbonate compositions containing metal salts of sulfonic acids of aromatic sulfides. A cyclic polycarbonate oligomer is disclosed in U.S. Pat. No. 4,751,283. The oligomer may be prepared from monomers containing aromatic disulfides. Also relevant is pending U.S. patent application Ser. No. 07/891,875 filed Jun. 3, 1992, which disclosed a stabilizer entailing an aromatic disulfide. Also relevant is the co-pending patent application Ser. No. 08/018,623(Mo-3913) filed Feb. 17, 1993 which disclosed a stabilizing agent for polycarbonate comprising a particular aliphatic disulfide optionally in conjunction with a polyether polyol. A key advantage of the present invention resides in the lower volatility of the polymeric version of the disulfide esters than low molecular weight compounds.